Apparatus and method for cleaning glass substrates using a cool hydrogen flame

ABSTRACT

Contaminants from surfaces of temperature sensitive substrates, such as glass substrates are removed by exposing the surfaces to a hydrogen Surface-mixed diffusion flame for a predetermined duration of time. The predetermined duration of time being insufficient to heat up the surfaces substantially thereby causing damage to the temperature sensitive substrates.

FIELD OF THE INVENTION

The present invention relates in general to the surface cleaning ofglass substrates for Flat Panel Displays. More particularly, the presentinvention relates to an apparatus and method for using a cool hydrogenflame for surface cleaning of glass substrates for manufacturing of FlatPanel Displays.

BACKGROUND OF THE INVENTION

Flat panel displays (FPDs) are well known devices commonly used inindustrial and consumer devices. The most common FPDs are the LiquidCrystal Displays (LCDs) and the Plasma Display Panels (PDPs). The FPDshave relatively small footprints as compared to Cathode Ray Tube (CRT)displays. They consume less energy. The images produced are notdistorted at the edges. FPDs are currently used in almost every field oftechnology that requires a graphic display. They are fast replacing CRTmonitors as the de facto output peripheral for personal computers. FPDsare also displacing the conventional CRT television sets.

One of the important elements of the FPDs is the glass substrates whichare used to make the viewable screen of the displays. The qualityrequirements of such glass substrates are very high. They have to be ofhigh clarity, high degree of flatness, uniform thickness and free fromcontaminants. These glass substrates are now manufactured in largepieces measuring about 890 mm by 680 mm with a thickness of about 0.7mm. They are later cut into smaller pieces for their variousapplications.

FPD manufacturers often rely on service providers to perform thecleaning task and to deliver the ready-to-use glass substrates accordingto their requirements to reduce their overhead costs. The typicalstandard of cleanliness that is required of glass substrates of the size890 mm by 680 mm is approximately 15,000 particles of size 0.3 micron.

In a glass substrate cleaning plant, the manufactured glass substratesare delivered in packages of 10-12, held together by four slotted anglesmade from urethane foam in order to protect each glass substrate fromanother. The surface of these glass substrates at this stage is highlycontaminated. The contaminants range from small general particulatematter like dust to other organic compounds and protein matter. Some ofthese organic compounds and protein matter can be attributed to humanhandling and can include natural oils from the skin or simply put“fingerprints”.

The washing or cleaning of glass substrates in the manufacturing of FPDsis crucial. It ensures that the surfaces of the glass substrates arefree from contaminants. This is necessary for the proper coatingrequirements according to the different types of FPDs. The importance ofcleaning the glass substrates is basically to ensure the clarity of theimages produced when the FPDs are finally assembled. Contaminantspresent on the surfaces of the glass substrates prevent proper coatingand causes distortion.

Some of the possible approaches or methods used in cleaning glasssubstrates are: high temperature de-ionized water, the RadianceProcess®, High Power Ultrasonic cleaning and Plasma cleaning.

The currently most practiced method of cleaning glass substrates is tofirst wash them with high temperature de-ionized (DI) water, followed byvacuum drying. The glass substrates are then conveyed directly into aclean room environment for packaging. This method though somewhateffective, consumes several million cubic meters of DI water for atypical production process. The use of vacuum drying also increasescosts, as power requirements are also very high. Studies have indicatedthat a plant cleaning 5000 FPD parts a day would require about 1.44cubic meters of DI water per FPD part. This washing method also becomesless effective when the particulate size approaches 0.3 microns.

The Radiance Process® makes use of a laser and a continuous laminar flowof an inert gas to remove contaminants. The laser breaks the bondsholding the contaminants to the glass substrate and the inert gascarries the contaminants away. Recent demonstrations have shown that thetotal particulate counts were reduced dramatically. However, theRadiance Process® is highly dependent on maintaining the laminar flow ofinert gases over the glass substrate surface. The equipment to provideand carry out this Radiance Process® is presently not available at a lowcost.

Another method used is High Power Ultrasonic cleaning. The technologyitself is not new. Ultrasonic cleaning has already been used withPrinted Circuit Boards. It has been applied here in the cleaning ofglass substrates as well. However, the method is far from perfect.During the ultrasonic cleaning process, cavitation of the cleaning fluidresults in formation of small bubbles impacting onto the surface of theglass substrates. This sometimes causes micro-fractures on the glasssurface and renders the glass substrate unusable.

Plasma Cleaning technology is performed in high vacuum systems byexposing the parts to be cleaned to energized plasmas. Though effective,plasma systems require high sources of energy to energize the plasmasand to provide the high vacuum environments. Invariably, such a systemmight be too expensive to use merely on account of cleaning the surfacesof a glass substrate.

There is thus a need for a novel, effective and economical method andapparatus for removing of contaminants from surfaces of glasssubstrates.

SUMMARY OF THE INVENTION

The present invention seeks to provide a method and apparatus forcleaning glass substrates using a cool hydrogen flame.

Contaminants from surfaces of temperature sensitive substrates, such asglass substrates are removed by exposing the surfaces to a hydrogenSurface-mixed diffusion flame for a predetermined duration of time. Thepredetermined duration of time being insufficient to heat up thesurfaces substantially thereby causing damage to the temperaturesensitive substrates.

In a first aspect, the present invention provides a method for removingcontaminants from surfaces of temperature sensitive substrates, themethod comprising the steps: producing a hydrogen Surface-mixeddiffusion flame, and exposing the hydrogen Surface-mixed diffusion flameto the surfaces for a predetermined duration of time, the predeterminedduration of time being insufficient to heat up the surfacessubstantially thereby causing damage to the temperature sensitivesubstrates.

In another aspect, the present invention provides a process for cleaningsurfaces of a glass substrate, the process comprising the steps: washingthe glass substrate with de-ionized water, drying the glass substrate,producing a hydrogen Surface-mixed diffusion fame; and exposing thehydrogen Surface-mixed diffusion flame to the surfaces for apredetermined duration of time, the predetermined duration of time beinginsufficient to heat up the glass substrate substantially therebycausing damage to the glass substrate.

In a further aspect, the present invention further provides a system forcleaning surfaces of a glass substrate for manufacturing Flat PanelDisplays, the system comprising: a jig for securely holding the glasssubstrate, a conveying means for transporting the jig, a washing unitusing de-ionized water for cleaning the glass substrate, a drying unit,and a cleaning chamber using Hydrogen surface-mixed diffusion flame.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top elevation plan view of the layout of the apparatusaccording to the present invention.

FIG. 2 shows a right side cross-sectional view of the Hydrogen FlameChamber according to section A-A of FIG. 1.

FIG. 3 shows a block diagram illustrating the steps of cleaning theglass substrates according to the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a system 10 according to the present invention for thecleaning of glass substrates for the manufacture of FPDs. The system 10comprises of conveying means 12 a,12 b, a jig 14, a DI water tank 16, adrying chamber 18, a Hydrogen cleaning chamber 20 and a clean-roompackaging facility 22. The Hydrogen cleaning chamber 20 is designed tobe a vacuum chamber. It operates at an internal pressure of about 3 psior 0.2 bar. To achieve this a vacuum pump 24 is used.

Referring to FIG. 2, the Hydrogen cleaning chamber 20 comprises astainless steel chamber 32, the vacuum pump 24 and a pair of HydrogenBurner Hubs 35 a,35 b. The jig 14 holds a glass substrate 30 in aposition substantially equidistant from the two Hydrogen Burner Hubs 35a,35 b.

The Hydrogen Burner Hubs 35 a/35 b are made up of a plurality oflinearly arranged burner nozzles. The length of the Hydrogen Burner Hubis about that of the length of the glass substrate to be cleaned. Theflames produced by the burner nozzles are gentle, “cool” and cleanburning. They are produced by burning pure hydrogen in pure oxygen.

However, the hydrogen and oxygen are not pre-mixed before being ignited.Instead, the burner nozzle is of a tube in orifice design. That is tosay, a main nozzle ejecting oxygen gas, and a central nozzle positionedinside the main nozzle ejecting hydrogen gas. When the burner nozzle isin operation, hydrogen gas is being ejected with an envelope of oxygengas surrounding it. When this hydrogen gas is ignited, it burns only atthe envelope of the hydrogen and oxygen gas interface. This produces aflame referred to as a Surface-mixed Diffusion Flame 40 a,40 b. It canbe described as a “cool” burning flame as its temperature is much lowerthan the conventional pre-mixed flames. To ensure complete combustion ofthe hydrogen gas, a 100% excess of oxygen is ejected from the nozzles.Since the Hydrogen cleaning chamber 20 is a vacuum chamber operating atabout 3 psi, there is no opportunity for the gases to accumulate even ifthey are not used up.

Referring to FIG. 1 and FIG. 3, the steps of the process 100 of cleaninga glass substrate 30 starts with the mounting 110 of the glass substrate30 onto a jig 14. The glass substrate 30 is then conveyed to a DI watertank 16. The glass substrate 30 is lowered or dipped 115 into the DIwater tank for an initial washing. Any soluble contaminants and looseparticulate matter would be removed here. The glass substrate 30 is thenraised and conveyed to the drying chamber 18 for drying 120. Compressedair or dry steam is used to blow or displace any moisture on the glasssubstrate 30. This also aids in removal of other particulate matter. Theglass substrate is then conveyed to the Hydrogen Flame chamber 20 forthe final surface cleaning of the glass substrate using Hydrogen Flame125. Referring to FIG. 2, the Hydrogen Burner Hubs 35 a,35 b aresituated at a level near the top of the glass substrate 30 as it entersthe Hydrogen Flame Chamber 20. The Hydrogen Burner Hubs 35 a,35 b havealready been ignited The Surface-mixed Diffusion Flame 40 a, 40 b arejust in contact with the surfaces near the top of the glass substrate30. The jig 14 is then raised vertically allowing the flames to comeinto contact with the rest of the glass substrate 30. In effect, theflames are being gently swept over the entire surfaces of the glasssubstrate 30. The flames are “swept” over the surfaces at a rate ofabout 20 mm per second. This results in any contaminants being vaporizedfrom the surfaces of the glass substrate. At this rate, such a systemcould very well clean a single glass substrate in less than a minute,close to 1500 units in a single 24 hour day.

In this present system, it is the glass substrate that is being moved,while the burners remain stationery. It is also entirely possible, tohave the glass substrate remain stationery while the burners are beingarticulated or even having both being articulated in a manner to ensurethat the entire surfaces of the glass substrate are “swept”.

The cleaned glass substrate 30 is then immediately conveyed into aclean-room packaging facility 22. The glass substrate 30 is thenpackaged 130 in an inert gas to protect it from contamination andincrease its electrical stability. A process which is also known aspassivation. The glass substrates are now ready for delivery to the FPDmanufacturers.

Using hydrogen flames for the clean vaporization of contaminants is byno means a new technology. However, the product to be cleaned is a 890mm by 680 mm glass substrate with a thickness of about 0.7 mm. Aconventional pre-mixed hydrogen flame would simply be too hot. The glasssubstrate of such a large area would simply warp and lose its flatnessand clarity. If a pre-mixed flame was kept at a further distance, itwould simply not vaporized the contaminants. Only the Surface-mixedDiffusion flame is “cool” enough to be exposed directly to the glasssubstrate and yet still be able to vaporize the contaminants.

Furthermore, the well known properties of hydrogen flames is itsclean-burning effect which does not produce soot or carbon particleslike most hydrocarbon flames. The absence of soot during combustion alsosignificantly reduces the radiative heat release by hydrogen flames thusmaking it a “cool” flame. In addition, radicals produced during hydrogencombustion are extremely reactive and will effectively destroy andremove organic contaminants quickly.

To further increase the performance of the present invention, chemicalsadditives are introduced into the system 10 to enhance the cleaningprocess of the present invention. Chemicals additives such as hydrogenperoxide and ozone can increase the presence of radicals in the system10 and consequently increase the performance of the cleaning of theglass substrate 30 as radicals readily react with the organiccontaminants effectively destroying them.

Ozone is introduced into the DI water tank 16 such that the DI waterused is ozonized. The strong oxidizing properties of ozone in the DIwater is highly effective in removal of any organic contaminants on theglass substrates 30. The ozone can be introduced into the DI water tank16 by at least one gas nozzle introduced into the DI water tank 16.Correspondingly, the process 100 in accordance with the presentinvention would further comprise an additional step of introducing ozoneinto the DI water tank 16 before the glass substrate 30 is dipped intothe DI water tank 16 for initial washing. Washing the glass substrate 30in ozonized DI water further enhances the performance of the hydrogencleaning chamber 20.

In addition to the Surface-mixed Diffusion flame vaporizing thecontaminants on the glass substrates 30; radicals produced duringcombustion of hydrogen further ensure that organic contaminants presentwould be eliminated. This can further be enhanced by the additionalintroduction of hydrogen peroxide into the hydrogen cleaning chamber 20.

Hydrogen peroxide is introduced into the Hydrogen Cleaning Chamber 20 inthe from of a fine mist by a plurality of spray nozzles. The spraynozzles are preferably located near the Hydrogen Burner Hubs 35 a/35 band oriented such that the spray nozzles are pointing at the glasssubstrate being cleaned as well as the Surface-mixed Diffusion flame 40a,40 b. The spray nozzles could also be arranged such that the spraynozzles are co-axial to the Hydrogen Burner Hubs 35 a/35 b. The finemist of hydrogen peroxide directed at the glass substrate 30 beingcleaned results in more hydroxyl radicals being formed as the hydrogenperoxide dissociates due to the heat generated by the Surface-mixedDiffusion flame 40 a,40 b. Any water formed as a result of thedissociation of the hydrogen peroxide is converted into steam by theSurface-mixed Diffusion flame 40 a,40 b and removed by the vacuum pump24. The fine mist of hydrogen peroxide is also directed at the glasssubstrate 30 to also help in cooling the temperature of the glasssubstrate 30 to prevent any overheating which may result in the glasssubstrate 30 warping due to the heat from the Surface-mixed Diffusionflame 40 a,40 b. Correspondingly, the process 100 in accordance with thepresent invention further comprises an additional step of spraying afine mist of hydrogen peroxide directed at the glass substrate 30. Thisspraying of fine mist of hydrogen peroxide occurs simultaneously withthe cleaning of the glass substrate 30 using the the Surface-mixedDiffusion flame 40 a,40 b.

Alternatively, ozone is introduced into the Hydrogen Cleaning Chamber 20to enhance the performance of the present invention. The ozone isintroduced into the Hydrogen Cleaning Chamber 20 via a plurality of gasnozzles. The gas nozzles would be sited away from the Hydrogen BurnerHubs 35 a,35 b but directed towards the glass substrate 30 after theglass substrate has undergone the cleaning process by the Surface-mixedDiffusion. The ozone is directed at the glass substrate 30 just beforeit is conveyed to the clean-room packaging facility 22. The introducedozone dissociates to form oxygen radicals and oxygen gas. The radicalsaid further in the elimination of any remaining contaminants on theglass substrate 30 before the glass substrate 30 is packaged 130.

Furthermore, the process 100 of cleaning the glass substrate 30 wouldfurther comprise an additional step of directing ozone gas at the glasssubstrates 30 after the step of cleaning using Hydrogen Flame 125 andbefore the step of packaging 130 the glass substrate 30.

This method of using a Surface-mixed Diffusion flame advantageouslyovercomes the shortcomings of the other methods of cleaning. It usesmuch less water, less power consumption and does not require expensiveequipment. This method does not induce additional mechanical stress onthe glass substrate and does not compromise the quality requirements ofthe FPD manufacturers. The gases required are easily available and donot constitute a high cost. Some of the more stubborn contaminants like“fingerprints” comprising of natural oils and organic matter are easilyremoved compared to when using hot DI water.

The above description according to one example of the invention does notin any way limit the scope of the invention. It would be apparent to oneskilled in the art that the present invention may be modified or used inor with other types of temperature sensitive substrates withoutdeparting from the scope of the invention.

1. A method for removing contaminants from surfaces of temperaturesensitive substrates, said method comprising the steps: producing ahydrogen diffusion flame by burning hydrogen and oxygen at an interfaceformed by an envelope of oxygen surrounding the hydrogen, wherein thehydrogen and oxygen are not pre-mixed before being ignited; spraying afine mist of hydrogen peroxide towards said hydrogen diffusion flame andsaid temperature sensitive substrates; and exposing said hydrogendiffusion flame to said surfaces for a predetermined duration of time;wherein: said hydrogen diffusion flame generates radicals to react withsaid contaminants for removing said contaminants; and said predeterminedduration of time is insufficient to heat up said surfaces substantially,thereby not causing damage to said temperature sensitive substrates. 2.The method according to claim 1, wherein said method is performed in avacuum environment.
 3. The method according to claim 1, wherein saidtemperature sensitive substrate comprises one or more of the following:glass substrates and semiconductor silicon-based substrates.
 4. Themethod according to claim 1, said method further comprising: directingozone gas at said temperature sensitive substrate after said step ofexposing said hydrogen diffusion flame to said surfaces for apredetermined duration of time.
 5. The method according to claim 1,wherein the oxygen is at least 100% more than the hydrogen.
 6. A processfor cleaning surfaces of a glass substrate, said process comprising thesteps: washing said glass substrate with de-ionized water; drying saidglass substrate; producing a hydrogen diffusion fame by burning hydrogenand oxygen at an interface formed by an envelope of oxygen surroundingthe hydrogen, wherein the hydrogen and oxygen are not pre-mixed beforebeing ignited; spraying a fine mist of hydrogen peroxide towards saidhydrogen diffusion flame and said glass substrate; and exposing saidhydrogen diffusion flame to said surfaces for a predetermined durationof time; wherein: said hydrogen diffusion flame generates radicals toreact with contaminants for removing said contaminants; and saidpredetermined duration of time is insufficient to heat up said glasssubstrate substantially, thereby not causing damage to said glasssubstrate.
 7. The process according to claim 6, wherein said glasssubstrate is for the manufacturing of Flat Panel Displays.
 8. Theprocess according to claim 6, wherein said step of exposing saidhydrogen diffusion flame to said surfaces is carried out in a vacuumchamber.
 9. The process according to claim 6, wherein the oxygen is atleast 100% more than the hydrogen.
 10. The process according to claim 6,wherein said step of washing said glass substrate with de-ionized waterfurther comprises introducing ozone into said de-ionized water.
 11. Theprocess according to claim 6, said process further comprising: directingozone gas at said glass substrate after said step of exposing saidhydrogen diffusion flame to said surfaces.